In the 3GPP LTE project (3rd generation Partnership Project, Long Term Evolution) for cellular communication systems, an RLC (Radio Link Control) protocol is used for communication between users in a cell and the controlling node of the cell, i.e. the so called eNodeB, “evolved NodeB”.
In the RLC, traffic is sent as so called PDUs, i.e. Protocol Data Units, which are identified by being given sequence numbers. In response to PDUs from a transmitting party, the receiving party sends so called RLC status PDUs to the transmitting party, with so called ACKs and/or NACKS, i.e. acknowledgements (ACK) that data has been properly received, or information (NACK) that data has not been properly received, i.e. received only partly or not at all. The ACKs and NACKS in the RLC status PDUs are sent as PDU sequence numbers, in order to identify the PDU in question.
In LTE systems, RLC PDUs may be segmented, which has as a consequence that there will be two or more PDU segments with the same sequence number, since the sequence number is a property of the PDU. The process of segmenting PDUs is also denoted as re-segmentation.
Because of the re-segmentation in LTE, the sequence numbers will not suffice to identify the data for which ACK or NACK is sent.